Growing mint can be a double-edged sword. Because peppermint (Mentha x piperata) is such a useful herb in the kitchen, first dig up a small patch and plant it in a roomy container. Like other mints, peppermint is happy to grow in confined quarters. Also like other mints, peppermint spreads rapidly by sending out shallow rhizomes, which eagerly take over space where you want to grow other plants. To get it under control, dig out the plant material and dispose of it in the garbage. Sift through the soil with a digging fork to make sure you get out all the little pieces. Watch the area carefully for several months, and dig out any plants that pop up. Should bits of mint appear close to other plants you want to keep, clip off the tops and cover the roots with a flat stone, a board, or several layers of newspaper. Smothering peppermint plants weakens them, making them easier to pull up when the light-blocking covers are removed several weeks later. Keep up your control efforts for the rest of the season. Should more peppermint plants appear in spring (which they probably will), you can dig them out, smother them, or treat them with a glyphosate herbicide such as Roundup. Be careful with this or any other herbicide, because it can damage or kill a huge range of plants. When using herbicides to control mint or other weedy plants that grow near more desirable species, cover nearby cultivated plants to protect them from spray drift. Or use an old paintbrush to apply the herbicide directly to the plants you want to eliminate.